This application outlines a career development plan for me to become an independent investigator researching novel antidiarrheal mechanisms and therapeutic approaches for diarrhea in children. I am now in my first faculty position as an Assistant Professor of Pediatric Gastroenterology in the Department of Pediatrics at the University of Florida. I have a strong background in fluid, electrolyte and nutrition physiology, having received a Ph.D. in ion transport physiology at Karolinska Institute and postdoctoral training in GI physiology under the late Dr. Steven Hebert at Yale University, where I initiated my interest in the calcium/nutrient-sensing receptor (CaSR) in the gut. I am now working with my mentor, Dr. Mansour Mohamadzadeh, studying the unique antidiarrheal function of this nutrient sensing receptor. Acute infectious diarrhea is a worldwide problem, especially among infants, young children and immune-compromised patients. Sadly, 1.3 million children die each year, not because of the infections causing diarrhea, but due to the associated dehydration. The pro-absorptive Oral Rehydration Solution (ORS) is the only recommended oral therapy for children with acute diarrhea, yet it neither reduces diarrhea/secretion nor alters the overly activ enteric nerve (ENS) activity/motility - both significant contributors to diarrhea. The overarching question of this application is the following: can we develop a novel anti-diarrheal therapy that i as simple and has both pro-absorptive and anti-secretory properties while reducing ENS activity and motility? Based upon preliminary data, I hypothesize that the intestinal CaSR is a likely candidate for developing such a therapeutic. The preliminary data show that CaSR is expressed in the gut epithelium, both in the absorbing surface epithelium and secreting crypts. Also, CaSR is densely present in the ENS, both in the fluid-modulating submucosal plexus and the motility-modulating myenteric plexus. Furthermore, using isolated microperfused crypts, it was observed that activating the epithelial CaSR reversed enterotoxin-induced fluid movement from net secretion to net absorption. To test the hypothesis, the following specific aims have been developed: Aim 1: will define the ion transport mechanisms influenced by epithelial CaSR. These studies will be performed in ex vivo intestinal segments using classical physiological techniques that involve the use of Ussing chamber, pH stat titration and measurement of isotope fluxes. Aim 2: will describe influences on gut motility by neuronal CaSR by examining CaSR modulations of evoked contractions in intestinal muscle strips in vitro and gastro-intestinal transit in vivo. Lastly, Aim 3: will assess CaSR anti-diarrheal effect in vivo by examining CaSR effects in (a) enterotoxin (cholera toxin, E coli STa & Rotavirus NSP4)- and (b) Norovirus- induced diarrhea. It is anticipated that this project will yield new insights into intestinal physiology and may lead to the development of novel CaSR-based therapeutics for acute diarrhea. Abbreviations: Ca2+i, intracellular Ca2+; Ca2+o, extracellular Ca2+; CaSR, Ca2+-sensing receptor; CTX, cholera toxin; ENS, enteric nervous system; i.v., intravenously; i.p., intraperitoneally; NSP4, rotavirus non structure protein 4 enterotoxin; ORS, oral rehydration solution; p.o., per os; SCFA, short-chain fatty acids, STa, E. coli heat stable enterotoxin;